[IMP] base, model, website[*]: add get_base_url helper

Before this commit, when we want to make an external url, we need to know
if the record is related to a specific website or not, and choose base_url
or website domain.

Now each model have a fonction get_base_url (public to be callable from email
template, ...) and website override this function to add the website domain if
the record is website specific.

closes odoo/odoo#34089

Signed-off-by: Jérémy Kersten (jke) <jke@openerp.com>